It is known in the art of induction heating that a solid body placed in an induction field, such as that generated by an induction coil excited by high-frequency ac current, begins to heat on its surface. The induction field induces electrical currents to flow on the surface of the body. Heating is the result of power dissipated by the currents as they flow against the electrical resistance of the body. The heat generated on the surface of the body can then propagate into the interior of the body. Hence, this known form of induction heating may be thought of as heating the body from the outside in. This property of induction heating is widely used for the heat treatment of metals, for example.
However, it is sometimes desirable to heat the body from the inside out. One application of "inside out" heating is in the melting and refining of metals in a vacuum. Presently, vacuum melting and refining typically requires the metal which is to be melted and refined to be contained in a crucible, usually of a ceramic material. The metal is thus susceptible to contamination from the crucible.
This source of contamination can be eliminated if the metal body itself acts as its own crucible. If the interior of the metal body is heated while the outside of the body is kept cool, i.e., the body is heated from the inside out, the metal will not be forced to contact a containing crucible and, therefore, could be refined in extremely clean conditions.
Typical induction heating apparatus heat from the outside in, and are not suitable for this type of melting and refining. There is thus a need for an induction heating apparatus and method that heats from the inside out. This invention fulfills that need.